Solving some of the greatest problems facing humanity -- pandemics, climate change, access to clean water - will depend on a diverse STEM workforce creating socially just solutions. Thus, there is a need to broaden participation in STEM to bring diverse perspectives to bear on these complex problems. Broadening STEM participation relies on understanding how students identify with and envision futures in STEM. Middle school represents a critical time for students' identity development, and therefore, for their potential career exploration. In order to better understand the influence of identity and possible selves on middle school students' career exploration, this study examined middle school students' experiences of a STEM career exploration event and explored how students expressed their STEM identities and STEM possible selves in this context. I conducted a multiple case study, with data collection consisting of semi-structured and photo elicited interviews, surveys, classroom observations, and student-created videos. The findings of this study revealed that students leveraged personal interests and learning preferences to engage in STEM Career Day participatory structures. Students expressed STEM identities and possible selves that intersected with their ideas about STEM people and salient factors of whole-person identities. Notably, social identities were often overlooked or disregarded by students. This social identity colorblindness was identified as an area of future study because of the large body of prior research that indicates the significance of social identities in STEM. Implications drawn from these findings point to the importance of offering students active roles in designing, participating in, and reflecting on STEM career exploration events. Additionally, drawing on students' ideas about identities presents opportunities to support their expressions of and connections to their personal STEM identities and possible selves. Finally, researchers and educators should consider engaging students throughout the STEM career exploration event process in order to create more inclusive experiences that nurture and broaden students' ideas about possible selves in STEM. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]